One of the principal commercial processes employed to manufacture high density linear ethylene polymers is to polymerize ethylene in the presence of a chromium oxide catalyst supported on silica. While the catalysts employed in this process are characterized as being a chromium oxide supported on silica, it is believed that the chromium undergoes at least partial reaction with silicon atoms to form complex molecules whose precise chemical composition has not been established with certainty. It is believed that at least a portion of the chromium is present in the hexavalent state.
In a specific aspect of this process, the polymerization is carried out in a hydrocarbon medium having little or no solvent action on the resin being produced and the resin, as formed, precipitates as fine solid particles. For this reason, this particular process is known in the art as the Particle Form Process. As used throughout this specification, the term Particle Form Process will be restricted to a process carried out in the presence of a chromium oxide catalyst and carried out in a hydrocarbon medium having solubility characteristics such that the resin, as produced, precipitates in the form of fine solid particles.
One of the limitations of the Particle Form Process is that the resins produced by the process have a relatively narrow molecular weight distribution, and a relatively low melt flow shear ratio which conventionally is expressed as the ratio obtained by dividing the high load melt index (ASTM 1238-70, Condition F) by the normal load melt index (ADTM 1238-70, Condition E). For a number of industrial purposes, it is desirable to have available high density linear ethylene polymers having broad molecular weight distributions and high melt flow shear ratios.
Many workers have attempted to modify the Particle Form Process to expand its capability to manufacture ethylene polymers having broader molecular distributions and higher melt flow shear ratios. Such efforts have been directed principally to modifying the chromium oxide-supported catalysts employed in the process. The success of such efforts has been marginal, at best, and many workers in the art believe that the Particle Form Process inherently is restricted to the manufacture of ethylene polymers having narrow molecular weight distributions and low melt flow shear ratios.